It is desirable to operate elevators in a manner to provide the quickest floor-to-floor travel time consistent with acceptable levels of acceleration and jerk, as well as providing a suitably smooth ride. In traditional hydraulic elevators, a positive displacement pump is driven by a constant speed motor, and acceleration/deceleration of the car is controlled by valving of the hydraulic fluid. The acceleration and deceleration of the car varies with the load in the car and with temperature-dependent viscosity of the hydraulic fluid. Therefore, precise car speed profile control is impossible, and a time-wasting, slow creep speed is needed near each landing to ensure that the car will not overshoot the landing and that it can be brought to a controlled stop. The overall speed of these elevators is quite low compared to traction (rope driven) elevators, and the energy consumption is several times greater than that of traction elevators.
In the past few years, hydraulic elevators which emulate characteristics of conventional traction elevators have been attempted, utilizing variable speed hydraulic pumps driven by induction motors, controlled by variable voltage, variable frequency (VVVF) inverters utilizing car speed control algorithms responsive to car reference speed profile generators. Regulating the flow of hydraulic oil to control car speed allows much closer control over car acceleration and speed, and utilizes energy much more efficiently.
However, accurate control over car speed is hampered by temperature-responsive variations in volume and viscosity of hydraulic oil, and attendant variations in speed inaccuracies resulting from pump leakage. Furthermore, due to the inherent low rigidity (flexibility) of the related hydromechanical system (particularly where an offset hydraulic cylinder is utilized with a lifting rope), the car is subjected to low frequency resonant modes which increase peak values of required jerk and acceleration, and result in an uncomfortable, bumpy ride.
Prior attempts to reduce car vibration have utilized the integral of the difference between pump angular speed and car speed as a feedback input to the car speed controller. Attempts to compensate for car speed variations as a function of pump leakage have included correcting the commanded (or reference) car speed from the car speed profile generator by the integral of the difference between commanded car speed and actual car speed.
Heretofore, the use of closed loop elevator car controls has not proved adequate to the task due to low accuracy of the car speed control and instabilities in car speed induced by the controller.